Atmos. Chem. Phys., 11, 9343–9366, 2011 www.atmos-chem-phys.net/11/9343/2011/ Atmospheric doi:10.5194/acp-11-9343-2011 Chemistry © Author(s) 2011. CC Attribution 3.0 License. and Physics High-ozone layers in the middle and upper troposphere above Central Europe: potential import from the stratosphere along the subtropical jet stream T. Trickl1, N. Bartsch-Ritter¨ 2, H. Eisele1, M. Furger2, R. Mucke¨ 1, M. Sprenger3, and A. Stohl4,* 1Karlsruher Institut fur¨ Technologie, Institut fur¨ Meteorologie und Klimaforschung (IMK-IFU), Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany 2Paul Scherrer Institut, Labor fur¨ Atmospharenchemie,¨ 5232 Villigen PSI, Switzerland 3Eidgenossische¨ Technische Hochschule (ETH) Zurich,¨ Institut fur¨ Atmosphare¨ und Klima, Universitatsstr.¨ 16, 8092 Zurich,¨ Switzerland 4Lehrstuhl fur¨ Okoklimatologie,¨ Technische Universitat¨ Munchen,¨ Am Hochanger 13, 85354 Freising-Weihenstephan, Germany *now at: Norwegian Institute for Air Research, P.O. Box 100, Instituttveien 18, 2027 Kjeller, Norway Received: 28 July 2010 – Published in Atmos. Chem. Phys. Discuss.: 15 December 2010 Revised: 14 July 2011 – Accepted: 30 August 2011 – Published: 9 September 2011 Abstract. Specific very dry high-ozone layers, starting important mechanism for stratosphere-to-troposphere trans- roughly two days after the onset of high-pressure periods port (STT) and also confirm the model predictions of pro- during the warm season, have been reproducibly observed nounced and persistent STT along the subtropical jet stream. in the middle and upper troposphere with the ozone lidar in Garmisch-Partenkirchen (Germany). These episodes, previ- ously not understood, were recently analysed based on ex- 1 Introduction tending backward simulations with the FLEXPART particle dispersion model to as many as twenty days and on jet-stream Very dry air masses with peak ozone mixing ratios of 80 analyses including calculations with the LAGRANTO trans- to 150 ppb were consistently detected in all six ozone lidar port model. In all six cases analysed the model results indi- measurement series covering at least four days carried out cate ozone import from the stratosphere on an extremely long in Garmisch-Partenkirchen (Germany) during high-pressure path along the subtropical jet stream over the Pacific Ocean, periods between 1996 and 2001 (May to August). These spe- Asia and, in part, all the way back to the Atlantic Ocean. cific high-ozone phases typically started two days after the The analysis suggests that stratospheric influence is the most beginning of the respective high-pressure period and, in part, important factor for the increase in ozone and is related to lasted for several days, which indicates an important contri- rather shallow transfer of air from the stratosphere into the bution to the free-tropospheric ozone budget. These obser- upper- and mid-tropospheric air streams observed with the vations were not understood at all since they could be nei- lidar. Contributions from the boundary layers of East Asia ther interpreted by high ozone from the lower troposphere and North America are just occasionally present, in one case over North America nor by stratospheric air rapidly subsid- documented by a very dense aerosol plume from the Asian ing over North America or the Atlantic Ocean (Trickl et al., deserts. The considerable vertical and temporal extent of 2003). Two of these layers contained aerosols indicating many of these layers and peak ozone mixing ratios between the presence of some air from a remote planetary bound- 80 and 150 ppb suggest that the observations are related to an ary layer (PBL), most likely mixed into these ozone-rich air streams in the vicinity of frontal systems. A common feature is a coherent bundle in the ensemble of backward Correspondence to: T. Trickl trajectories initiated in these layers that stays in the mid- ([email protected]) dle and upper troposphere between the Pacific Ocean and Published by Copernicus Publications on behalf of the European Geosciences Union. 9344 T. Trickl et al.: High-ozone layers in the middle and upper troposphere Garmisch-Partenkirchen. However, these trajectories had tude. However, trans-Pacific transport may also occur almost never been followed back beyond a few degrees west of entirely in the upper troposphere. Very rapid trans-Pacific North America. transport of radon-rich air (half life 3.8 days) in the upper tro- Observations of similar very dry air streams with elevated posphere was reported on by Kritz et al. (1990) for a number ozone were reported, e.g., by Prados et al. (1999), Bithell et of measurements during summer months. Liang et al. (2007) al. (2000), Law et al. (2001) (see also Penkett et al., 2004) observed a case in which subsequent trans-Pacific and trans- and were also found by us in numerous vertical profiles North-American transport of an ozone-rich air mass took for North America extracted from the MOZAIC (Measure- place exclusively at high altitudes (2 August 2004). The ments of Ozone by Airbus In-Service Aircraft, Marenco et entire passage from the West Pacific to the North Ameri- al., 1998) data base. In some cases, elevated CO has been can east coast occurred within about eight days. A signifi- found and upper-tropospheric advection from the Pacific is cant ozone enhancement in the upper troposphere above mid- indicated. Newell et al. (1999), by analysing MOZAIC ver- latitude North America due to import from the stratosphere tical profiles, found that atmospheric layers are most fre- was concluded by Cooper et al. (2006). quently associated with an increase in ozone and a simul- The almost zonal propagation of air masses to the east taneous decrease in water vapour. Typical altitudes are 6 km at high altitudes described by Cooper et al. (2004a, b) and and the average thickness of layers showing this kind of anti- Liang et al. (2007) is similar to what we found for our ob- correlation is of the order of 1 km. servations by trajectory analyses. A very intriguing idea The occurrence of high ozone values in the dry layers seen for explaining the high ozone values in the dry air streams in our results could suggest the presence of a stratospheric observed above Garmisch-Partenkirchen would be that of component. But, obviously, the mechanism of stratospheric shallow intrusions directly injecting ozone into an upper- import leading to such high ozone values in such wide lay- tropospheric air stream over the Pacific Ocean or farther to ers must differ from the deep intrusions most commonly ob- the west. Sprenger et al. (2003) locate a maximum in occur- served at our observational site (e.g., Reiter et al., 1971; rence of shallow intrusions (partly exceeding a relative fre- Eisele et al., 1999; Stohl and Trickl, 1999; Trickl et al., 2003, quency of 30 %) between North Africa and the Pacific Ocean 2010; Zanis et al., 2003). These deep intrusions are mostly between 30◦ and 40◦ N. This transfer seems to occur in the confined to narrow air tongues with decreasing ozone mix- vicinity of the subtropical jet stream which is a rather persis- ing ratios as the layers approach the ground. The typical tent feature along this path, weakening in summer (Koch et ozone mixing ratios measured in stratospheric streamers at al., 2006; see also Chen, 1995; Dunkerton, 1995, who em- the nearby Zugspitze summit (2962 m a.s.l.) range just be- phasize the role of the monsoon circulation for stratosphere- tween 60 and 80 ppb. There is no way to explain the high troposphere exchange). In fact, Langford et al. (1998) and mid- and upper-tropospheric ozone concentrations in a layer Langford (1999), examining 5.5 years of lidar data of the several kilometres thick by an admixture of such a diluted air Fritz-Peak station (Colorado, USA), suggested that ozone en- mass. hancements observed between 6 and 12 km above this site are Potential source regions are the atmosphere above the Pa- correlated with stratosphere-to-troposphere transport (STT) cific Ocean and East Asia where frequent and pronounced associated with the subtropical jet stream. However, there stratospheric air intrusions may occur (Austin and Midgley, is limited direct observational material on the processes in- 1994; James et al., 2003; Sprenger et al., 2003). Cooper volved (e.g., Gouget et al., 1996; Cammas et al., 1998; et al. (2005) report on two strong intrusions in 2004 that Zachariasse, 2000; Zahn et al., 2002) and the importance reached Hawaii and led to the observation of 81 and 115 ppb of the subtropical jet stream for the overall STT budget had ozone at the Mauna Loa Observatory (3400 m a.s.l.), respec- long been ignored (e.g., Gouget et al., 1996). Zachariasse tively. The second event (10 March 2004) even reached the et al. (2000) hypothesize shear-induced differential advec- sea level, 60 ppb were recorded at Honolulu. This is particu- tion or clear-air turbulence to be responsible for the STT larly remarkable since, at least over Europe, stratospheric in- events causing mid-tropospheric maxima in their sonde pro- trusions rarely proceed to the ground (e.g., Reiter et al., 1987; files rather than tropopause folding. Davis and Schuepbach, 1994; Elbern et al., 1997; Eisele et The idea of potential inflow of stratospheric air from al., 1999; Schuepbach et al., 1999). the region of shallow intrusions described by Sprenger et Cooper et al. (2004b) also describe that air from a dry in- al. (2003) as a source of the enhanced ozone, indeed, soon trusion may be mixed into the ascending air streams of ad- became a strong motivation for the study presented here. A jacent warm conveyor belts (WCBs). In their case study (10 particular challenge has been the enormously long travel of to 11 May 2002) they find that roughly half of the strato- these air masses.